Simplification of the Crithidia assay for biopterin has progressed to where an efficient, fully defined medium is at last on hand, mainly by use of a slightly acidic medium with a lowered heme content, thanks to the discovery that most of the heme served as a Fe 3 ion transporter rather than satisfying the absolute requirement for exogenous heme; such compounds as 2,3-dihydroxybenzoic or sulfosalicylic acid now supplement heme as Fe 3 ion transporter. The goal now clearly in sight -- to make the assay easily feasible for non-microbiological laboratories -- is rendered all the more desirable by the widening scope of the biopterin assay, clinically and theoretically. We intend to extend our findings that serum biopterin is significantly elevated in gout patients, predictable (other things equal) from guanosine triphosphate being a precursor of biopterin. Our finding, that chromaffin tissue (dried whole suprarenal tissue, neuroblastoma cells) is about 20-fold higher in biopterin than control tissues and cells, offers the possibility that the biopterin assay might serve for early diagnosis of chromaffin-cell tumors, e.g., neuroblastomas and phaeocytomas, and, if so, for monitoring therapy -- a new consideration now that experimental chemotherapy of such tumors is beginning. Reports that patients with rheumatoid arthritis have significantly lowered urinary biopterin, and that some cases of phenylketonuria result from deficiency in the biopterin cofactor of phenylalanine hydroxylase rather than deficiency of the apoenzyme, suggest applications of the assay as clinical material becomes available. BIBLIOGRAPHIC REFERENCES: Baker H, Frank O, Bacchi CJ, Hutner SH. 1974. Biopterin content of human and tissues determined protozoologically. Amer. J. Clin. Nutr. 27: 1247-53.